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// Copyright 2017 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// This implements the write barrier buffer. The write barrier itself
// is gcWriteBarrier and is implemented in assembly.
// See mbarrier.go for algorithmic details on the write barrier. This
// file deals only with the buffer.
// The write barrier has a fast path and a slow path. The fast path
// simply enqueues to a per-P write barrier buffer. It's written in
// assembly and doesn't clobber any general purpose registers, so it
// doesn't have the usual overheads of a Go call.
// When the buffer fills up, the write barrier invokes the slow path
// (wbBufFlush) to flush the buffer to the GC work queues. In this
// path, since the compiler didn't spill registers, we spill *all*
// registers and disallow any GC safe points that could observe the
// stack frame (since we don't know the types of the spilled
// registers).
package runtime
import (
// testSmallBuf forces a small write barrier buffer to stress write
// barrier flushing.
const testSmallBuf = false
// wbBuf is a per-P buffer of pointers queued by the write barrier.
// This buffer is flushed to the GC workbufs when it fills up and on
// various GC transitions.
// This is closely related to a "sequential store buffer" (SSB),
// except that SSBs are usually used for maintaining remembered sets,
// while this is used for marking.
type wbBuf struct {
// next points to the next slot in buf. It must not be a
// pointer type because it can point past the end of buf and
// must be updated without write barriers.
// This is a pointer rather than an index to optimize the
// write barrier assembly.
next uintptr
// end points to just past the end of buf. It must not be a
// pointer type because it points past the end of buf and must
// be updated without write barriers.
end uintptr
// buf stores a series of pointers to execute write barriers
// on. This must be a multiple of wbBufEntryPointers because
// the write barrier only checks for overflow once per entry.
buf [wbBufEntryPointers * wbBufEntries]uintptr
// debugGen causes the write barrier buffer to flush after
// every write barrier if equal to gcWorkPauseGen. This is for
// debugging #27993. This is only set if debugCachedWork is
// set.
debugGen uint32
const (
// wbBufEntries is the number of write barriers between
// flushes of the write barrier buffer.
// This trades latency for throughput amortization. Higher
// values amortize flushing overhead more, but increase the
// latency of flushing. Higher values also increase the cache
// footprint of the buffer.
// TODO: What is the latency cost of this? Tune this value.
wbBufEntries = 256
// wbBufEntryPointers is the number of pointers added to the
// buffer by each write barrier.
wbBufEntryPointers = 2
// reset empties b by resetting its next and end pointers.
func (b *wbBuf) reset() {
start := uintptr(unsafe.Pointer(&b.buf[0])) = start
if writeBarrier.cgo || (debugCachedWork && (throwOnGCWork || b.debugGen == atomic.Load(&gcWorkPauseGen))) {
// Effectively disable the buffer by forcing a flush
// on every barrier.
b.end = uintptr(unsafe.Pointer(&b.buf[wbBufEntryPointers]))
} else if testSmallBuf {
// For testing, allow two barriers in the buffer. If
// we only did one, then barriers of non-heap pointers
// would be no-ops. This lets us combine a buffered
// barrier with a flush at a later time.
b.end = uintptr(unsafe.Pointer(&b.buf[2*wbBufEntryPointers]))
} else {
b.end = start + uintptr(len(b.buf))*unsafe.Sizeof(b.buf[0])
if (*unsafe.Sizeof(b.buf[0])) != 0 {
throw("bad write barrier buffer bounds")
// discard resets b's next pointer, but not its end pointer.
// This must be nosplit because it's called by wbBufFlush.
func (b *wbBuf) discard() { = uintptr(unsafe.Pointer(&b.buf[0]))
// empty reports whether b contains no pointers.
func (b *wbBuf) empty() bool {
return == uintptr(unsafe.Pointer(&b.buf[0]))
// putFast adds old and new to the write barrier buffer and returns
// false if a flush is necessary. Callers should use this as:
// buf := &getg().m.p.ptr().wbBuf
// if !buf.putFast(old, new) {
// wbBufFlush(...)
// }
// ... actual memory write ...
// The arguments to wbBufFlush depend on whether the caller is doing
// its own cgo pointer checks. If it is, then this can be
// wbBufFlush(nil, 0). Otherwise, it must pass the slot address and
// new.
// The caller must ensure there are no preemption points during the
// above sequence. There must be no preemption points while buf is in
// use because it is a per-P resource. There must be no preemption
// points between the buffer put and the write to memory because this
// could allow a GC phase change, which could result in missed write
// barriers.
// putFast must be nowritebarrierrec to because write barriers here would
// corrupt the write barrier buffer. It (and everything it calls, if
// it called anything) has to be nosplit to avoid scheduling on to a
// different P and a different buffer.
func (b *wbBuf) putFast(old, new uintptr) bool {
p := (*[2]uintptr)(unsafe.Pointer(
p[0] = old
p[1] = new += 2 * sys.PtrSize
return != b.end
// wbBufFlush flushes the current P's write barrier buffer to the GC
// workbufs. It is passed the slot and value of the write barrier that
// caused the flush so that it can implement cgocheck.
// This must not have write barriers because it is part of the write
// barrier implementation.
// This and everything it calls must be nosplit because 1) the stack
// contains untyped slots from gcWriteBarrier and 2) there must not be
// a GC safe point between the write barrier test in the caller and
// flushing the buffer.
// TODO: A "go:nosplitrec" annotation would be perfect for this.
func wbBufFlush(dst *uintptr, src uintptr) {
// Note: Every possible return from this function must reset
// the buffer's next pointer to prevent buffer overflow.
// This *must not* modify its arguments because this
// function's argument slots do double duty in gcWriteBarrier
// as register spill slots. Currently, not modifying the
// arguments is sufficient to keep the spill slots unmodified
// (which seems unlikely to change since it costs little and
// helps with debugging).
if getg().m.dying > 0 {
// We're going down. Not much point in write barriers
// and this way we can allow write barriers in the
// panic path.
if writeBarrier.cgo && dst != nil {
// This must be called from the stack that did the
// write. It's nosplit all the way down.
cgoCheckWriteBarrier(dst, src)
if !writeBarrier.needed {
// We were only called for cgocheck.
// Switch to the system stack so we don't have to worry about
// the untyped stack slots or safe points.
systemstack(func() {
if debugCachedWork {
// For debugging, include the old value of the
// slot and some other data in the traceback.
wbBuf := &getg().m.p.ptr().wbBuf
var old uintptr
if dst != nil {
// dst may be nil in direct calls to wbBufFlush.
old = *dst
wbBufFlush1Debug(old, wbBuf.buf[0], wbBuf.buf[1], &wbBuf.buf[0],
} else {
// wbBufFlush1Debug is a temporary function for debugging issue
// #27993. It exists solely to add some context to the traceback.
func wbBufFlush1Debug(old, buf1, buf2 uintptr, start *uintptr, next uintptr) {
// wbBufFlush1 flushes p's write barrier buffer to the GC work queue.
// This must not have write barriers because it is part of the write
// barrier implementation, so this may lead to infinite loops or
// buffer corruption.
// This must be non-preemptible because it uses the P's workbuf.
func wbBufFlush1(_p_ *p) {
// Get the buffered pointers.
start := uintptr(unsafe.Pointer(&_p_.wbBuf.buf[0]))
n := ( - start) / unsafe.Sizeof(_p_.wbBuf.buf[0])
ptrs := _p_.wbBuf.buf[:n]
// Poison the buffer to make extra sure nothing is enqueued
// while we're processing the buffer. = 0
if useCheckmark {
// Slow path for checkmark mode.
for _, ptr := range ptrs {
// Mark all of the pointers in the buffer and record only the
// pointers we greyed. We use the buffer itself to temporarily
// record greyed pointers.
// TODO: Should scanobject/scanblock just stuff pointers into
// the wbBuf? Then this would become the sole greying path.
// TODO: We could avoid shading any of the "new" pointers in
// the buffer if the stack has been shaded, or even avoid
// putting them in the buffer at all (which would double its
// capacity). This is slightly complicated with the buffer; we
// could track whether any un-shaded goroutine has used the
// buffer, or just track globally whether there are any
// un-shaded stacks and flush after each stack scan.
gcw := &_p_.gcw
pos := 0
for _, ptr := range ptrs {
if ptr < minLegalPointer {
// nil pointers are very common, especially
// for the "old" values. Filter out these and
// other "obvious" non-heap pointers ASAP.
// TODO: Should we filter out nils in the fast
// path to reduce the rate of flushes?
obj, span, objIndex := findObject(ptr, 0, 0, !usestackmaps)
if obj == 0 {
if span.isFree(objIndex) {
// For gccgo, it is possible that we have a write barrier
// writing to unintialized stack memory. So we could see
// a bad pointer in the write barrier buffer. Don't mark
// it in this case.
// TODO: Consider making two passes where the first
// just prefetches the mark bits.
mbits := span.markBitsForIndex(objIndex)
if mbits.isMarked() {
// Mark span.
arena, pageIdx, pageMask := pageIndexOf(span.base())
if arena.pageMarks[pageIdx]&pageMask == 0 {
atomic.Or8(&arena.pageMarks[pageIdx], pageMask)
if span.spanclass.noscan() {
gcw.bytesMarked += uint64(span.elemsize)
ptrs[pos] = obj
// Enqueue the greyed objects.